For stabilizing the spine, pedicle screws are inserted in the spine and their heads are interconnected by a rod. In order to maintain a dynamic behavior, elastic rods are known as an alternative to rigid metallic rods.
Since elastic rods are often manufactured from plastic materials, and plastic materials are generally susceptible to creep, plastic rods cannot simply be clamped by a press-fit as this is easily possible, for example, with metal rods.
Therefore, for a reliable long-term clamping of plastic rods (i.e. made of polymeric material), particular devices are required.
Only few methods to this end are known in the art so far. These will be explained below and their disadvantages as compared to the present invention will be pointed out.
The inventions according to patent specifications EP 1 364 622 B1 (Freudiger) and EP 1 527 742 B1 (Freudiger) show positive or form-fit anchorages. The rod and its seat in the screw head are provided with geometrically mutually fitting grooves. However, positioning the grooved surfaces requires precise introduction to avoid canting. Furthermore, grooved surfaces do not allow continuous positioning.
The invention according to patent specification EP 1 759 646 B1 (Freudiger) shows a frictional anchorage with an indirect form-fit. The connecting rod is smooth and is clamped between two members, the screw head and a laterally guided spacer, whose contour nominally deviates from the rod contour in the area of contact. Therefore, this nominal contour deviation locally penetrates into the rod such that high stress intensities result in the clamped areas and relative movements due to shear deformation in the non-clamped areas. Furthermore, rod material may flow off between the clamping areas if they do not form a closed cage.